Capacitive touch panel input device that measures internal resistances for compensation

Abstract

An input device includes a selection circuit to which a plurality of electrodes disposed in a capacitive touch panel are connected, a line connected to the selection circuit and having a same internal resistance as that between the plurality of electrodes and the selection circuit, an A/D conversion part connected to the selection circuit so as to measure a voltage of the electrodes or a measurement value when the line is connected thereto, wherein the electrodes and the line are each connected selectively to the A/D conversion part by the selection circuit, and a control part configured to control the selection circuit so as to allow the A/D conversion part to measure a voltage of one of the electrodes and the measurement value, and to calculate an electrostatic capacitance of one of the electrodes based on the voltage of one of the electrodes and the measurement value.

What is claimed is: 
     
       1. An input device, comprising:
 a selection circuit to which a plurality of electrodes disposed in a capacitive touch panel are connected through a first line; 
 a second line connected to the selection circuit and having a same internal resistance as that between the plurality of electrodes and the selection circuit wherein the second line is separate from the first line connecting the plurality of electrodes to the selection circuit; 
 a constant current source configured to supply electric power of constant current to the plurality of electrodes; 
 an A/D conversion part connected to the selection circuit so as to either measure a voltage of the plurality of electrodes when the first line is connected thereto or to measure a measurement value when the second line is connected thereto, wherein the plurality of electrodes and the second line are each connected selectively to the A/D conversion part by the selection circuit; 
 a control part configured to control the selection circuit so as to allow the A/D conversion part to measure a voltage of one of the plurality of electrodes and the measurement value when the second line is connected to the A/D conversion part, and to calculate an electrostatic capacitance of one of the plurality of electrodes based on the voltage of one of the plurality of electrodes and the measurement value; and 
 a third line connected to the selection circuit and comprising a corrective resistance and the same internal resistance as that between the plurality of electrodes and the selection circuit, 
 wherein said second line is connected to a ground at one end thereof, 
 wherein the A/D conversion part is configured to measure a measurement value when the corrective resistance is connected to the A/D conversion part, and 
 wherein the control part is configured to subtract a value due to an influence of the internal resistance from the measurement value when the corrective resistance is connected to the A/D conversion part, subtract the value due to the influence of the internal resistance from the voltage of one of the plurality of electrodes, based on the measurement value when the second line is connected to the A/D conversion part, and calculate the electrostatic capacitance of one of the plurality of electrodes. 
 
     
     
       2. The input device according to  claim 1 , wherein the control part controls the selection circuit so as to allow the A/D conversion part to measure the electrostatic capacitance of one of the plurality of electrodes by connecting only the one of the plurality of electrodes to the A/D conversion part. 
     
     
       3. The input device according to  claim 1 , wherein the internal resistance comprises a first internal resistance between the plurality of electrodes and the selection circuit and a second resistance in the selection circuit. 
     
     
       4. The input device according to  claim 3 , wherein the control part controls the selection circuit so as to allow the A/D conversion part to measure a voltage between the first internal resistance and the second internal resistance.